1 Thessalonians 4:13 But I would not have you to be ignorant, brethren, concerning them which are asleep, that ye sorrow not, even as others which have no hope. 14 For if we believe that Jesus died and rose again, even so them also which sleep in Jesus will God bring with him. 15 For this we say unto you by the word of the Lord, that we which are alive and remain unto the coming of the Lord shall not prevent them which are asleep. 16 For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first: 17 Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord. 18 Wherefore comfort one another with these words.

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Few riddles in the Bible are more divisive and puzzling than this one. Much of what Jesus prophesied however was clearly fulfilled by the destruction of Jerusalem in A.D. 70, which fulfilled many of God's judgments on Israel, and did indeed occur within the generation Jesus was speaking to. That it was a generation as a single human lifespan can be determined by how the Greek word genea translated generation here is used in Matthew 1:17.[2]

The following is a full breakdown of the controversial Matthew, Mark, and Luke passages up to the phrases in question:

Beginning:

Jesus leaves the temple and tells His disciples it will be utterly destroyed. This was fulfilled by the destruction of Jerusalem in A.D. 70. (Mt. 24:1-2; Mk. 13:1-2; Lk. 21:5-6)

Jesus' disciples ask Jesus privately when this destruction will occur, and what the sign of His coming and the world's end will be (two separate questions). Perhaps they assumed the temple would not be destroyed until the world's end and thought they were just asking one big question, a mistake. (Mt. 24:3; Mk. 13:3-4; Lk. 21:7)

When the destruction would occur:

Jesus tells them to be careful they aren't deceived, many false prophets will claim to be Christ and deceive many. This was fulfilled shortly afterwards as well by proclaimed false Messiahs such as Simon of Peraea, Athronges, Menahem Ben Judah, Vespasian, and Simon bar Kohba. (Mt. 24:4-5,11; Mk. 13:5-6; Lk. 21:8)

Jesus tells them wars and rumors of wars will occur, with nations fighting nations, pestilences, famines, and earthquakes occurring all over as the "beginning of sorrows". Possible wars alluded to could be Rome's invasion of Britain, constant Jewish uprisings including the famous destruction of Jerusalem, warring between Otho and Vitellius for control of Rome, Rome's war with the Kingdom of Dacia, and China's famous warring later culminating in The War of the Three Kingdoms.[3] At any rate, this could be considered fulfilled within a generation as well. (Mt. 24:6-8; Mk. 13:7-8; Lk. 21:9-11)

Jesus warns His followers will be delivered up to be tortured and killed; hated by all nations because of His name with betrayal and severe affliction occurring. This of course was very much fulfilled by the Roman Empire's persecution of the early Church within a generation. (Mt. 24:9-10; Mk. 13:9,12-13) Additional detail given is that Christians should not premeditate what they will say beforehand but say what the Holy Ghost leads them to in such cases. (Mk. 13:11) In the Luke account it is specified that "before all these" would occur such torture, so before all the wars the killing of Christians would come. Luke contains much more detail here, including a final phrase, "But there will not a hair on your head perish. In your patience possess your souls." (Lk. 21:12-19)

Because of the great evil the love of many will grow cold, but those which endure will be saved. This too could be considered fulfilled within a generation. (Mt. 24:12-13)

When the end will come:

Jesus says the Gospel must be preached in all the world as a witness to all nations, and only then will the end come. Even today, there remain areas of the world that have not yet heard the Gospel. (Mt. 24:14; Mk. 13:10)

Jesus advises those in Israel who see the Abomination of Desolation prophesied in Daniel standing in the temple calling himself God to flee into the mountains and not return for their belongings. This is paralleled by the prophecies in Revelation of the Beast. (Mt. 24:15-20; Mk. 13:14-18) Luke contains a lot of extra detail that "these are the days of vengeance, that all things which are written will be fulfilled". Jesus here also states that the Jews will be led away captive into all nations and Jerusalem trodden down until the "times of the Gentiles" are fulfilled. This last prophecy is just coming to completion, since Jews were split among all nations until 1947 when the United Nations made Israel a country once more. (Lk. 21:20-23)

Jesus says then there will be "Great Tribulation" above anything like it in the world's history and the days are shortened for the sake of the elect, otherwise none would be saved. (Mt. 24:21-22; Mk. 13:19-20)

Jesus emphasizes that false prophets will arise performing miracles so great they might even to an extent deceive the elect, but not to believe claims of secret messiahs since Jesus' return will be as obvious as the lightning in the sky even as eagles gather over a carcase. (Mt. 24:23-28; Mk. 13:21-22)

Jesus emphasizes "take heed; behold, I have foretold you all things". (Mk. 13:23)

Jesus says only after the tribulation of those days will the sun and moon darken, the stars fall from heaven, and He return. The nations of Earth will mourn His coming and He will gather His elect "from one end of Heaven to the other". (Mt. 24:29-31; Mk. 13:24-27; Lk. 21:25-28)

Jesus uses the analogy of the fig tree to conclude His discussion of the end, saying such signs will evidence the coming of the end. (Mt. 24:32-33; Mk. 13:28-30; Lk. 21:29-31)

Conclusion

Jesus says "this generation shall not pass, till all these things be fulfilled." He states Heaven and Earth will pass away but not His words. He appears to be referring to the first question asked Him about when the destruction of Jerusalem would occur. (Mt. 24:34-35)

Jesus says of the day and hour of His return no man knows, not even Himself, only God the Father. He now is clearly addressing the second question He'd been asked, when the end of the world and His return would occur. (Mt. 24:36-37)

What most probably do not notice when seeing such an apparent contradiction is that Jesus was indeed answering at least two different questions, (1) When the destruction of Jerusalem's Temple He spoke of would occur, and (2) What the signs of His coming and the end of the world would be. Thus His answer referred to two very separate, distinct points in history, one to come soon (the destruction of Jerusalem) and one to come much later (His return and the end of the world).

Given 1 Thessalonians 4:13-18 it appears the disciples themselves may have been confused and expected Jesus' return to occur within their own lifetimes, given that some were worried Jesus would return for them after some of them had died. If so that was because they inappropriately assumed they were asking one question about the destruction of Jerusalem, Christ's return, and the end of the world, when really they were asking at least two.

At any rate the prophecy above seems quite valid and divided into two parts, one which was fulfilled in the generation Jesus spoke of by the destruction of Jerusalem and Rome's persecution of Christians, and the later prophecies echoed in the book of Revelation about Jesus' return and the end of the world that have yet to be fulfilled. Some signs, like the Gospel being preached in all nations, the Jews returning to Israel after being scattered through all nations, and the temple of Jerusalem being rebuilt where the Beast/Abomination of Desolation will enter, are all very near to being fulfilled. When these are fulfilled it will indicate the end of the world and Jesus' return are imminent.